A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The carbonylation catalysts typically contain rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. In addition to methanol, the reaction medium also comprises methyl acetate, water, methyl iodide and the catalyst.
The crude acetic acid product from the reactor is processed in a purification system to remove impurities. These impurities, which may be present in trace amounts, affect the quality of acetic acid, especially as the impurities are circulated through the reaction process. These impurities include saturated and unsaturated carbonyl compounds generally referred to as permanganate reducing compounds (PRCs). PRCs may include, for example, compounds such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde and the like, and the aldol condensation products thereof.
Such PRCs may be removed through one or more distillation columns. For example, U.S. Pat. Nos. 6,143,930 and 6,339,171, the entireties of which are incorporated herein by reference, disclose two step distillation methods for producing high purity acetic acid wherein a light phase of the intermediate process stream, e.g., light phase of the distillate from a light ends column, is distilled in a first distillation column. This light phase of the process stream comprises primarily water along with various concentrations of methyl acetate, methanol, methyl iodide, di-methyl ether, acetic acid and acetaldehyde, and the first distillation column concentrates the PRCs, and in particular the acetaldehyde in the overhead stream thereof. The PRCs and alkyl iodides are then removed from the process stream in a second distillation tower containing a vertically-arranged stack of vapor-liquid contact tray assemblies in the rectification and stripping sections. Vapor-liquid contact tray assemblies are used in mass transfer or exchange columns to facilitate contact between, for example, upwardly flowing vapor streams and downwardly flowing liquid streams. Each tray assembly comprises a tray panel, a downcomer, and weir. The tray panels are disposed within the column to provide a horizontal surface across which the liquid streams may flow. Most of the area of the tray panels includes a pattern of apertures to allow vapor to flow upwardly through the tray panel for interaction with liquid flowing across the top surface of the tray panel. The area of the tray panel containing the apertures is referred to as the active tray area because vapor-liquid interaction occurs in this area.
In operation, the aforementioned light phase of the process stream is fed into a middle portion of the distillation column and flows in cascading fashion over the bottom trays which form the stripping section of the column. The vapor that boils off from the feed rises to the top of the column, where it is collected and condensed and re-admitted as reflux over the top trays of the column which form the rectification section of the column.
One objective of the second distillation column is to recover >98% of the entering feed component acetaldehyde with the distillate stream. This is accomplished by varying the reflux to feed ratio and the column bottom temperature at a fixed pressure. Another objective is to have as high a production rate as possible. As both feed rates and reflux rates are increased, it has now been discovered that the hydraulics capability of the top (rectification) and bottom (stripping) sections within the distillation column diverge, and become unbalanced. Specifically, the top section pressure drop increases substantially linearly with increasing hydraulic (vapor and liquid) loads. By contrast, the bottom section pressure drop increases in a rapid exponential manner causing it to become flooded, and thereby upsetting the entire column operation. In particular, it has now been observed that the flooding in the stripping section is caused by foaming of the liquid feed on the bottom trays due to an insufficient open area in the top downcomer region of both the top and bottom trays.
One way to avoid the foaming problem and to increase production would be to completely replace the existing trays of the distillation column with new trays sized to provide downcomers with larger areas. However, such a solution would be expensive, requiring a re-design of the trays, the custom manufacturing of all of the components of the re-designed trays, the complete removal of the old trays and the installation of the new ones. Additionally, a substantial amount of downtime would be required to remove all of the existing trays and install new ones. Another solution might be to adjust the temperatures, pressures and reflux to feed ratio within the distillation column to handle a larger rate of feed and reflux flow while avoiding the foaming on the trays that causes the flooding. However, such adjustments in operating parameters would result in a greater amount of impurities in the final product, thus jeopardizing the objective of recovering >98% of the entering feed component acetaldehyde with the distillate stream.
The need exists for distillation columns and processes for using such distillation columns capable of handling high rates of feed and reflux flow which reduces or eliminates tray flooding and the high costs associated with a complete re-design and replacement of the existing trays.